This invention relates to a star coupler using unequal optical signal distributing devices.
A plurality of nodes are interconnected via star couplers in a fiber optics transmission network. For detailed information on star couplers, see, for example, E. G. Rawson et all., IEEE Transactions on Communications, Vol. COM-26, No. 7, July 1978, pp. 983-990, "Fibernet: Multimode Optical Fibers for Local Computer Networs".
FIG. 4 shows an exemplary layout of a star coupler proposed by the applicant. The star coupler has a single unit of 1.times.2 equal optical distributor 3, four units of 1.times.2 unequal optical distributor 4 and two units of 2.times.2 unequal optical distributor 5 formed on a substrate 2. The respective components are interconnected via waveguides 11-18. Five optical fibers 6 are connected to the substrate 2 and an optical signal 1 is supplied to and delivered from the star coupler via these optical fibers 6. Shown by R in FIG. 4 is the radius of curvature of each waveguide. The numerals put before the terms "distributor" denote the numbers of optical paths provided at opposite ends of each component; for instance, "1.times.2" signifies the provision of one optical path at one end and two optical paths at the other end.
FIG. 5 shows an exemplary layout of the 1.times.2 unequal optical distributor 4. The waveguide 11 before branching has a width W.sub.1 and light is branched between waveguides 12 and 13 having different widths W.sub.2 and W.sub.3, respectively. The widths W.sub.2 and W.sub.3 are set at such values that the required distribution ratio will be attained. For instance, in order to achieve a branching ratio of 1:2, the widths W.sub.1, W.sub.2 and W.sub.3 should be set 40 .mu.m, 13 .mu.m and 27 .mu.m, respectively.
FIG. 6 shows an exemplary layout of the 2.times.2 unequal optical distributor 5 that is composed of a 1.times.2 equal optical distributor 9 and a 1.times.2 unequal optical distributor 10. Two waveguides 14a and 14b at one end are designed to have the same width W.sub.4 whereas two waveguides 15 and 16 at the other end are designed to have different widths W.sub.5 and W.sub.6.
FIG. 7 shows an exemplary layout of the 1.times.2 equal optical distributor 3. Waveguides 17a and 17b between which light is to be branched have the same width. Shown by reference numeral 18 is a waveguide before branching.
Referring to the star coupler shown in FIG. 4, the optical signal 1 supplied from terminal P.sub.1 via optical fiber 6 passes through the two units of 1.times.2 unequal optical distributor 4 to have part of it distributed to terminal P.sub.2. Thereafter, the signal passes through the 2.times.2 unequal optical distributors 5 to have part of it distributed to terminals P.sub.3 and P.sub.4. The remaining part of the signal is transmitted to terminal P.sub.0 via the 1.times.2 equal optical distributor 3. Symbol "x.sub.n " in FIG. 4 denotes the light component to be supplied to the star coupler and "y.sub.n " denotes the component to be delivered from the coupler.
As shown in FIG. 8, when signal S is supplied to the terminal P.sub.1 of the star coupler shown in FIG. 4, signal component bS is distributed to a specified terminal P.sub.0 whereas signal components aS are distributed to the other three terminals P.sub.2, P.sub.3 and P.sub.4. No part of the signal is distributed to the terminal P.sub.1 itself. Symbols a and b denote coefficients of distribution and bS&gt;aS&gt;0. When signal S is supplied to terminal P.sub.2, P.sub.3 or P.sub.4, the signal is distributed under the same fashion.
As shown in FIG. 9, when input signal S is supplied to the specified terminal P.sub.0, the signal is distributed equally among the four terminals P.sub.1, P.sub.2, P.sub.3 and P.sub.4 but no part of the signal is distributed to the terminal P.sub.0 per se.
The relationships of signal distribution in FIGS. 8 and 9 may be expressed by a matrix as follows: ##EQU1## where x.sub.0, x.sub.1, x.sub.2, x.sub.3 and x.sub.4 are input signals to the terminals P.sub.0, P.sub.1, P.sub.2, P.sub.3 and P.sub.4, respectively, whereas y.sub.0, y.sub.1, y.sub.2, y.sub.3 and y.sub.4 are output signals from those terminals.
A star coupler of the type described above in which all elements on the main diagonal (i=j) of the transfer matrix are zero but not all of the other elements are equal is designated hereinafter as a "half coupler". A fiber optics communication network is composed by combining the above-described half coupler with a "full coupler", in which all elements on the main diagonal of the transfer matrix are zero and all other elements are approximately of equal values. In the following description, the number of terminals in a half coupler shall be expressed in terms of the number of all terminals except a specified terminal whereas the number of terminals in a full coupler shall be expressed in terms of the total number of terminals present.
FIG. 10 shows a network comprising two 4-terminal half couplers HC1 and HC2, and a single 4-terminal full coupler FC. Input signal S to one terminal P.sub.1a of half coupler HC1 is distributed in such a way that bS accounting for greater part of the signal power is sent to a specified terminal P.sub.0a, with the remaining signal components being sent to the other terminals P.sub.2a, P.sub.3a and P.sub.4a. Since terminal P.sub.0a is connected to a terminal of full coupler FC, a signal equal to (1/3)bS is distributed to the other three terminals of that full coupler FC as shown in FIG. 10.
Another terminal of full coupler FC is connected to a specified terminal P.sub.0b of the other half coupler HC2. From this terminal, signal aS equal to 1/4.times.(1/3)bS=bS/12 is distributed to each of the other terminals P.sub.1b, P.sub.2b, P.sub.3b and P.sub.4b of the half coupler HC2. If a=b/12, the ratio of signal distribution among the terminals P.sub.2a, P.sub.3a and P.sub.4a can be made equal to the ratio of signal distribution among the terminals P.sub.1b, P.sub.2b, P.sub.3b and P.sub.4b.
For the technical rationale of combining half couplers with full couplers, the major feature is that with the ratio of signal distribution by a star coupler being designed appropriately, star couplers with a smaller number of terminals can be combined to realize the construction of a large-size network in a simple manner.
In the foregoing description, it has been assumed that the loss of optical signal is negligible. In practice, however, the transmission loss of optical fibers, the connection loss of connectors and the excessive loss of individual couplers are by no means negligible. The total insertion loss is the excessive loss added to the attenuation (insertion loss) caused for attaining the intended branching ratio.
FIG. 11 schematically shows an actual system that implements the network shown in FIG. 10. Node N.sub.1a in FIG. 11 corresponds to terminal P.sub.1a in FIG. 10, and node N.sub.1b in FIG. 11 corresponds to terminal P.sub.1b in FIG. 10. Node N.sub.1a, half coupler HC1, full coupler FC, half coupler HC2 and node N.sub.1b are interconnected via optical connectors PC.sub.1 to PC.sub.8 and optical fibers 6 extending for a total distance of 1 km.
Suppose here that the network implemented with the system shown in FIG. 11 suffers a transmission loss of -2 dB/km the optical fibers 6, a connection loss of -0.2 dB per site of optical connectors PC.sub.1 to PC.sub.8, and an excessive loss of -2 dB in each coupler. FIG. 12 is a level diagram of signal light as it travels from terminal P.sub.1a (corresponding to the transmission port of node N.sub.1a) to terminal P.sub.1b (corresponding to the transmission port of node N.sub.1b). With the light intensity of transmission output being normalized as zero dB, an attenuation of -3.6 dB (=-1 d-2 dB-0.2 dB.times.3: loss B.sub.1 occurring in zone A.sub.1) occurs in half coupler HC1 and in three optical connectors PC.sub.1, PC.sub.2 and PC.sub.3, then an attenuation of -8.4 dB (=-4 dB-2 dB- 0.2 dB.times.2-2 dB: loss B.sub.2 occurring in zone A.sub.2) occurs in full coupler FC, two optical connectors PC.sub.4 and PC.sub.5, and in the 1-km optical fibers 6, and a further attenuation of -8.4 dB (=-5.8 dB-2 dB-0.2 dB.times.3: loss B.sub.3 occurring in zone A.sub.3) occurs in half coupler HC2 and in three optical connectors PC.sub.6, PC.sub.7 and PC.sub.8. Thus, signal S being supplied to terminal P.sub.1a or half coupler HC1 leaves half coupler HC2 at each of the terminals P.sub.1b, P.sub.2b, P.sub.3b and P.sub.4b as signal aS that has decayed to -20.4 dB less than the initial signal. It should be noted that the -1 dB drop occurring in half coupler HC1, the -4 dB drop occurring in full coupler FC and the -5.8 dB drop occurring in half coupler HC2 are the amounts of ideal attenuation that should take place in the respective portions. Since the signal distributed to each of the terminals P.sub.2a, P.sub.3a and P.sub.4a of half coupler HC1 must have the same optical level as signal aS delivered from each of the terminals P.sub.1b, P.sub.2b, P.sub.3b and P.sub.4b of half coupler HC2, coefficient a of signal distribution by half coupler HC1 must be desired in such a way that signal S supplied to terminal P.sub.1a of half coupler HC1 decays by -20.4 dB before it is delivered to terminals P.sub.2a, P.sub.3a and P.sub.4a of the same half coupler HC1. For the sake of simplicity, let assume that signal S is attenuated by -20 dB; it then follows that the distribution coefficient a must be 1/100, namely the branching ratio must be 100:1.
As already described with reference to FIGS. 5 and 6, the unequal branching device which is designed to provide a predetermined branching ratio by adjusting the ratio between the widths of waveguides; however, this involves difficulty in attaining a very large branching ratio. Consider, for example, the case of achieving an attenuation of -20 dB; to this end, the ratio between the widths of waveguides must be adjusted to 100:1 but this means that the wider waveguide has a width of about 40 .mu.m whereas the narrower one has a width of about 0.4 .mu.m. As a matter of fact, fabricating such an extremely fine waveguide is very difficult from the viewpoint of manufacturing technology. Furthermore, such waveguide whose width is narrower than the wavelength of signal light can not guide the light any longer.
If structural constraints require that waveguides be curved, the general practice is to increase the radius of curvature as much as possible in order to suppress the radiation loss that would otherwise occur on account of the curvature of the waveguides. This has made it necessary to use substrate of a larger area for providing waveguides, inevitable leading to bulky equipment.
Furthermore, the four-terminal half coupler shown in FIG. 4 has the optical fiber 6 connected to each of the four end facets of the substrate 2; hence, in order to achieve optical coupling between the substrate 2 and each optical fiber 6, the four end faces of the substrate 2 have to be polished, thus increasing the time required to complete the fabrication process.